1. Field Of The Invention
This invention relates to the real time, three dimensional imaging of the anatomy,--ie. a real time 3D medical ultrasound machine, and 3D imaging in general.
2. Description Of The Prior Art
The above mentioned patents and patent applications (ie. mentioned in CROSS-REFERENCES TO RELATED APPLICATIONS) discuss 3D real time imaging and imaging using ellipsoidal backprojection. The following is a discussion of prior art, much of which discussion is common to the applicant's previous patents and patent applications:
The reconstruction technique described in U.S. Pat. No. 4,706,499 is essentially the backprojection of the echo samples over ellipsoids of revolution as will be more fully described in this application. The backprojections may be weighted as a function of the reconstruction point position to compensate for transmitter or receiver radiation patterns and other phenomena.
This imaging system can be implemented with nearly all commonly used types of transmitted pulses. The transmitted pulses that the imaging system may use also includes pulses with peaked autocorrelation functions that have a very small value except when the shift variable is near zero. These types of pulses will be termed "non interfering" or "interference free" for purposes of this application as there is little constructive and destructive interference and therefore strong grating lobes will not be formed when using a sparse array. A wideband white noise pulse is an extreme example. These types of pulses also can propagate relatively uniformly through a wide solid angle. Further discussion of these types of pulses may be found in "Random Data:Analysis and Measurement Procedures" by Bendat and Piersol. Periodic, oscillating, "interfering" pulses of a particular class may also be used for imaging if additional echo processing occurs before image reconstruction (such as echo time history convolution with a matched filter impulse response) or without additional processing if some image degradation is allowable. The pulses must be of short enough duration to allow adequate lateral and range resolution. Thus, a pulse of several sinusoidal cycles may be used if the total pulse duration, or length, is of the same order as the required resolution.
Ellipsoidal Backprojection is a method for the active imaging of a three dimensional volume using a single transmitted pulse or greatly reduced number of transmitted pulses and is discussed in detail in the previously mentioned patents and patent applications. Referring to FIG. 1, a short pulse of energy is transmitted which radiates outward, as an expanding sphere, through a wide solid angle. Echoes are received by a sparse array of receiver elements and, typically, then digitized into echo samples. These samples (which may be filtered first) are then backprojected over ellipsoids through the image by one means or another. The results constitute, basically, the reconstructed image, although additional processing steps may be implemented.
Ellipsoidal Backprojection is a linear image reconstruction method, although the point spread function varies with location of the reconstruction point. The point spread function is the image of a point object as reconstructed by the imaging system. In a linear imaging system, the final, reconstructed image is, essentially, the convolution of the point spread function with the original object to be imaged. This is well known and is described in: Introduction to Fourier Optics--Goodman; Linear Systems, Fourier Transforms, and Optics--Gaskill; or The Fourier Transform and its Application to Optics--Duffiuex.
The point spread function determines the imaging capabilities of a linear imaging system. Ellipsoidal Backprojection alone can yield an adequate point spread function for some applications, however, a particular type of linear filter, the "inverse triplet filter" (discussed in detail here), may be applied to the echo time histories, before backprojection, which greatly improves the resolution while substantially reducing the sidelobe levels. The resulting imaging system is still linear.